Radio frequency identification monitoring of stents

ABSTRACT

A method and system of monitoring environmental exposure of stents using radiofrequency identification is disclosed.

CROSS-REFERENCE

This application is a divisional application of U.S. application Ser.No. 11/486,688, filed on Jul. 13, 2006, which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to monitoring environmental exposure of stentsusing radiofrequency identification.

2. Description of the State of the Art

This invention relates to radially expandable endoprostheses, which areadapted to be implanted in a bodily lumen. An “endoprosthesis”corresponds to an artificial device that is placed inside the body. A“lumen” refers to a cavity of a tubular organ such as a blood vessel.

A stent is an example of such an endoprosthesis. Stents are generallycylindrically shaped devices, which function to hold open and sometimesexpand a segment of a blood vessel or other anatomical lumen such asurinary tracts and bile ducts. Stents are often used in the treatment ofatherosclerotic stenosis in blood vessels. “Stenosis” refers to anarrowing or constriction of the diameter of a bodily passage ororifice. In such treatments, stents reinforce body vessels and preventrestenosis following angioplasty in the vascular system. “Restenosis”refers to the reoccurrence of stenosis in a blood vessel or heart valveafter it has been treated (as by balloon angioplasty, stenting, orvalvuloplasty) with apparent success.

The treatment of a diseased site or lesion with a stent involves bothdelivery and deployment of the stent. “Delivery” refers to introducingand transporting the stent through a bodily lumen to a region, such as alesion, in a vessel that requires treatment. “Deployment” corresponds tothe expanding of the stent within the lumen at the treatment region.Delivery and deployment of a stent are accomplished by positioning thestent about one end of a catheter, inserting the end of the catheterthrough the skin into a bodily lumen, advancing the catheter in thebodily lumen to a desired treatment location, expanding the stent at thetreatment location, and removing the catheter from the lumen.

In the case of a balloon expandable stent, the stent is mounted about aballoon disposed on the catheter. Mounting the stent typically involvescompressing or crimping the stent onto the balloon. The stent is thenexpanded by inflating the balloon. The balloon may then be deflated andthe catheter withdrawn. In the case of a self-expanding stent, the stentmay be secured to the catheter via a retractable sheath or a sock. Whenthe stent is in a desired bodily location, the sheath may be withdrawnwhich allows the stent to self-expand.

The stent must be able to satisfy a number of mechanical requirements.First, the stent must be capable of withstanding the structural loads,namely radial compressive forces, imposed on the stent as it supportsthe walls of a vessel. Therefore, a stent must possess adequate radialstrength. Radial strength, which is the ability of a stent to resistradial compressive forces, is due to strength and rigidity around acircumferential direction of the stent. Radial strength and rigidity,therefore, may also be described as, hoop or circumferential strengthand rigidity.

Once expanded, the stent must adequately maintain its size and shapethroughout its service life despite the various forces that may come tobear on it, including the cyclic loading induced by the beating heart.For example, a radially directed force may tend to cause a stent torecoil inward. Generally, it is desirable to minimize recoil.

In addition, the stent must possess sufficient flexibility to allow forcrimping, expansion, and cyclic loading. Longitudinal flexibility isimportant to allow the stent to be maneuvered through a tortuousvascular path and to enable it to conform to a deployment site that maynot be linear or may be subject to flexure. Finally, the stent must bebiocompatible so as not to trigger any adverse vascular responses.

The structure of a stent is typically composed of scaffolding thatincludes a pattern or network of interconnecting structural elementsoften referred to in the art as struts or bar arms. The scaffolding canbe formed from wires, tubes, or sheets of material rolled into acylindrical shape. The scaffolding is designed so that the stent can beradially compressed (to allow crimping) and radially expanded (to allowdeployment). A conventional stent is allowed to expand and contractthrough movement of individual structural elements of a pattern withrespect to each other.

Additionally, a medicated stent may be fabricated by coating the surfaceof either a metallic or polymeric scaffolding with a polymeric carrierthat includes an active or bioactive agent or drug. Polymericscaffolding may also serve as a carrier of an active agent or drug.

Furthermore, it may be desirable for a stent to be biodegradable. Inmany treatment applications, the presence of a stent in a body may benecessary for a limited period of time until its intended function of,for example, maintaining vascular patency and/or drug delivery isaccomplished. Therefore, stents fabricated from biodegradable,bioabsorbable, and/or bioerodable materials such as bioabsorbablepolymers should be configured to completely erode only after theclinical need for them has ended.

A stent can be exposed to range of environmental conditions duringmanufacturing and storage, or generally, during the periodic of timebetween completion of manufacturing and implantation. The properties ofpolymeric stents or polymeric coatings on stents can be particularlysensitive to environmental conditions such as temperature, humidity,vibration, and shock. Exposure to extremes in such conditions cannegatively affect, for example, mechanical properties and drug delivery.

SUMMARY

Certain embodiments of the present invention include a method ofmonitoring a stent comprising: method of monitoring a stent comprising:obtaining readings of an environmental parameter from a sensor adjacentto a stent, the sensor positioned within or on a container including thestent; and transmitting the readings from an RFID tag located within oron the container to a transceiver, wherein a maximum tolerance of thestent for the environmental parameter is stored on the RFID tag; andcomparing the readings to the maximum tolerance for the environmentalparameter.

Other embodiments of the present invention include a method ofmonitoring a stent comprising: method of monitoring a stent comprising:obtaining readings of an environmental parameter from a sensor adjacentto a stent, the sensor positioned within or on a container including thestent, wherein the readings are received by an RFID tag located withinor on the container; and comparing the readings received by the RFID tagto a maximum tolerance for the environmental parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a stent.

FIG. 2 depicts an exemplary RFID system for monitoring a stent in acontainer.

FIG. 3 depicts a schematic embodiment of an e-beam sterilization system.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the present invention relate to monitoringconditions of polymeric implantable medical devices, particularlystents, during manufacturing, storage, and transportation. Theconditions of a stent can be monitored using Radio FrequencyIdentification (RFID) technology. RFID technology is known in the art. Asignificant advantage of the present invention is that it allowsmonitoring, storage, and analysis of the environmental exposure of aspecific stent during all or part of the period of time between lasercutting to implantation.

The method and systems described herein may be may be applied ingenerally to implantable medical devices. The methods and systems areparticularly relevant, for reasons discussed below, to implantablemedical devices having a polymeric substrate, a polymer-based coating,and/or a drug-delivery coating. A polymer-based coating may contain, forexample, an active agent or drug for local administration at a diseasedsite. An implantable medical device may include a polymer or non-polymersubstrate with a polymer-based coating.

Examples of implantable medical devices include self-expandable stents,balloon-expandable stents, stent-grafts, grafts (e.g., aortic grafts),artificial heart valves, cerebrospinal fluid shunts, pacemakerelectrodes, and endocardial leads (e.g., FINELINE and ENDOTAK, availablefrom Guidant Corporation, Santa Clara, Calif.). The underlying structureor substrate of the device can be of virtually any design. A non-polymersubstrate of the device may be made of a metallic material or an alloysuch as, but not limited to, cobalt chromium alloy (ELGILOY), stainlesssteel (316L), high nitrogen stainless steel, e.g., BIODUR 108, cobaltchrome alloy L-605, “MP35N,” “MP20N,” ELASTINITE (Nitinol), tantalum,nickel-titanium alloy, platinum-iridium alloy, gold, magnesium, orcombinations thereof “MP35N” and “MP20N” are trade names for alloys ofcobalt, nickel, chromium and molybdenum available from Standard PressSteel Co., Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel,20% chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20%nickel, 20% chromium, and 10% molybdenum.

The structure of a stent in particular can have a scaffolding or asubstrate that includes a pattern of a plurality of interconnectingstructural elements or struts. FIG. 1 depicts an example of a view of astent 100. Stent 100 has a cylindrical shape and includes a pattern witha number of interconnecting structural elements or struts 110. Ingeneral, a stent pattern is designed so that the stent can be radiallycompressed (crimped) and radially expanded (to allow deployment). Thestresses involved during compression and expansion are generallydistributed throughout various structural elements of the stent pattern.The present invention is not limited to the stent pattern depicted inFIG. 1. The variation in stent patterns is virtually unlimited. A stentsuch as stent 100 may be fabricated from a polymeric tube or a sheet byrolling and bonding the sheet to form a tube. A stent pattern may beformed on a polymeric tube by laser cutting a pattern on the tube.Representative examples of lasers that may be used include, but are notlimited to, excimer, carbon dioxide, and YAG. In some applications theuse of a femtosecond laser may be preferred. In other embodiments,chemical etching may be used to form a pattern on a tube.

A stent has certain mechanical requirements that are crucial tosuccessful treatment. For example, a stent must have sufficient radialstrength to withstand structural loads, namely radial compressiveforces, imposed on the stent as it supports the walls of a vessel. Inaddition, the stent must possess sufficient flexibility to allow forcrimping, expansion, and cyclic loading. Bending elements 130, 140, and150, in particular, are subjected to a great deal of stress and strainduring use of a stent.

The underlying structure or substrate of a stent can be completely or atleast in part made from a biodegradable polymer or combination ofbiodegradable polymers, a biostable polymer or combination of biostablepolymers, or a combination of biodegradable and biostable polymers.Additionally, a polymer-based coating for a surface of a device can be abiodegradable polymer or combination of biodegradable polymers, abiostable polymer or combination of biostable polymers, or a combinationof biodegradable and biostable polymers.

However, the mechanical properties of polymers are particularlysensitive to changes in temperature. In particular, as the temperatureincreases, there is an increased susceptibility of polymer chains tomove or rearrange which can negatively alter mechanical properties.Specifically, as the temperature approaches and surpasses the glasstransition temperature, Tg, polymer chain rearrangement increasesdramatically. Rearrangement of polymer chains is also time dependent.Thus, exposure of a polymer to a selected temperature and the amount oftime a polymer is exposed to the selected temperature are important inassessing changes in polymer properties.

Tg is the temperature at which the amorphous domains of a polymer changefrom a brittle vitreous state to a solid deformable or ductile state atatmospheric pressure. In other words, the Tg corresponds to thetemperature where the onset of segmental motion in the chains of thepolymer occurs. When an amorphous or semicrystalline polymer is exposedto an increasing temperature, the coefficient of expansion and the heatcapacity of the polymer both increase as the temperature is raised,indicating increased molecular motion. As the temperature is raised theactual molecular volume in the sample remains constant, and so a highercoefficient of expansion points to an increase in free volume associatedwith the system and therefore increased freedom for the molecules tomove. The increasing heat capacity corresponds to an increase in heatdissipation through movement. Tg of a given polymer can be dependent onthe heating rate and can be influenced by the thermal history of thepolymer. Furthermore, the chemical structure of the polymer heavilyinfluences the glass transition by affecting mobility.

As the temperature of a polymer sample approaches Tg physical agingincreases. Physical aging of a polymer refers to densification in theamorphous regions of a semi-crystalline polymer. Densification is theincrease in density of a material. Physical aging results in an increasein brittleness of a polymer which can result in cracking of struts uponcrimping and deployment. Many polymers have Tg's low enough to allowsignificant physical aging or densification to occur during the timeframe of storage, which can be a few days, a month, 3 months, 6 months,or more than 6 months. Even for polymers with Tg's above ambienttemperatures, significant physical aging can occur during storage.Polymeric stents generally are stored below ambient temperatures toreduce or prevent physical aging.

Additionally, environmental conditions can also affect a drug coating ona stent. Temperature and humidity can influence the release rate of drugfrom a drug coating. If the environmental moisture or the temperature ishigh enough before the stent is implanted (e.g., during storage), muchof the drug can be lost before implantation, changing the concentrationof the drug resident on the stent. Drugs can also degrade at increasedtemperatures during manufacture and storage conditions, altering thetotal content and release rate of the drug. A drug may also be degradedduring storage due to exposure to degradation agents present in theenvironment such as oxygen (air), light, or water (humidity, moisture).

Therefore, it would be advantageous to have available a system andmethod to monitor and record of the exposure of a stent to environmentalconditions. The record could extend from the end stages of themanufacturing process, such as laser cutting, through implantation.

In some embodiments, a method of monitoring a device, such as a stent,may include obtaining environmental parameter readings from a sensoradjacent to a stent, the sensor positioned within or on a containerincluding the stent. The readings may be transmitted from an RFID tagpositioned in or on the container to an RFID transceiver. An RFID tag istypically an integrated circuit that can receive, transmit, or storedata. The environmental parameters can include, but are not limited to,temperature, humidity, oxygen, light, vibration, and shock. Sensors formeasuring these parameters are known in the art and are commerciallyavailable. For example, sensors for measuring temperature, humidity,vibration, and shock can be obtained from Savi Technology in Sunnyvale,Calif.

In certain embodiments, the RFID tag and the sensor are integrated.Instrumental Inc. of Leeds, England has developed a technology that hasbeen named “Super RFID” which incorporates sensing using an RFID tag.Super RFID can be a sensor network or sensor telemetry. Sensor networkscan be used to monitor conditions and record that data, and ifnecessary, set off an alert if a condition moves beyond a certaincriteria.

Typically an RFID system includes a transceiver, scanning antenna, andRFID transponder or tag. The transceiver controls the antenna andcollects data. The scanning antenna which transmits a Radio Frequency(RF) signal under control of the transceiver, receives an RF signal backfrom the RFID Transponder that is passed back to the transceiver. TheRFID Transponder or RFID Tag contains the data that is being read and isnormally located on or within a package containing a product to bemonitored.

The scanning antenna transmits a Radio Frequency (RF) signal. The RFsignal accomplishes two tasks: it provides a passive RFID tag with theenergy to communicate and it provides a communication path between theRFID tag and the transceiver. When an RFID tag passes thru theelectromagnetic field of the scanning antenna, it detects the presenceof the RF signal and transmits the information contained in itsmicrochip which in turn is picked up by the scanning antenna and thedata is provided to the transceiver. If the RFID is a “passive” device,it also draws power from the magnetic field and uses this to power thecircuits of the RFID tag.

There are three kinds of RFID tags: active, passive, and semi-passive.Active RFID tags have their own internal power source which is used topower any ICs that generate the outgoing signal. Many active tags havepractical ranges of hundreds of meters, and a battery life of up to 10years. Passive RFID tags have no internal power supply. The minuteelectrical current induced in the antenna by the incoming radiofrequency signal provides just enough power for the integrated circuit(IC) in the tag to power up and transmit a response. Passive RFID tagsare limited in the physical distance they can transmit their data.Semi-passive RFID tags are very similar to passive tags except for theaddition of a small battery. This battery allows the tag IC to beconstantly powered, which removes the need for the aerial to be designedto collect power from the incoming signal.

In some embodiments, a stent is packaged and stored in a container. Thecontainer may be designed to inhibit, prevent, or significantly minimizeexposure of the stent to environmental conditions such as moisture,light, oxygen, etc. The stent may be packaged in the container before orafter sterilization of the stent. A stent can be sterilized byradiation, such as electron beam (e-beam) or by a suitable sterilizationfluid such as ethylene oxide.

FIG. 2 depicts an exemplary RFID system 200 for monitoring a stent 205in a sealed container 210. Container 210 can be provided for storage orpackaging of a stent carrying a drug. Stents are typically sterilized,packaged, stored, and transported in a “ready to implant” configurationin which the stent is disposed at the distal end of a catheter. A stentcan be crimped over a balloon. The stent-catheter system can be packagedprior to or after radiation sterilization. In one commerciallydistributed embodiment, container 210 also holds a balloon catheterassembly having stent 205 crimped onto a balloon.

Container 210 for a stent can be designed in any convenient form orshape that permits the effective enclosure of a stent or stent-catheterassembly contained therein. Container 210, however, should be compactand shaped so as to minimize storage space occupied by the container.For example, without limitation, container 210 can be in the shape of atube, box or a pouch. In one commercially useful embodiment, container210 can have a rectangular cross-section with a width between 8 in and12 in and a length between 10 in and 13 in. Also, depending on the typesof substance(s) used to construct container 210, container 210 can be ofvarious degrees of rigidity or flexibility. Container 210 can beconstructed of flexible films rather than rigid materials because it isless likely that the seal would be compromised by a change inatmospheric conditions during storage. For example, container 210 can beconstructed of two sheets or lamina which have been joined along anedge. Also, container 210 can be constructed of a single sheet or laminawhich has been folded and sealed along all edges or along all non-foldedges; or a bag or pocket which is sealed along one or more edges. Thepouches can be made from a polymer, glass, ceramic, metallic substance,or a combination thereof. Typically, the pouches are made of metallicfoil.

System 200 includes a sensor 215 configured to measure an environmentalparameter. Sensor 215 can be coupled to container 210 either in or oncontainer 210, depending upon the parameter to be measured. Sensor 215can be coupled to container by, for example, gluing or taping. Sensor215 can also be coupled to the stent, as described in U.S. Patent Appl.Pub. 2005016537.

Sensor 215 can measure the temperature, humidity, oxygen, light,vibration, or shock. To measure the exposure of stent 205 to humidity oroxygen, sensor 215 should be placed within container 210. Container 210can include any number of sensors corresponding to the number ofenvironmental variables to be measured.

Container 210 can be stored individually from or stored together withother packaged stents. For example, container 210 can be disposed in abox, such as chipboard box, along with a number of similar or identicalcontainers 210 including stents. The sensor and RFID tag in eachcontainer is configured to monitor exposure of a specific stent in therespective container.

Sensor 215 is configured to measure the temperature to which stent 205is exposed. In general, a sensor 215 can be located close enough tostent 205 to obtain a reading of the temperature to which the stent isexposed. Specifically, sensor 215 can be positioned less than 0.5 mm, 1mm, 2 mm, 5 mm, or 8 mm from stent 205.

As shown in FIG. 2, an RFID tag 220 is coupled to container 210. RFIDtag 220 is configured to receive sensor data from sensor 215. RFID tag220 can also transmit data to transceiver 225 which transmits the sensordata to storage and processing system 230. Sensor 215 and RFID tag 220are communicatively coupled as shown by line 217. Sensor 215 and RFIDtag 220 can communicate through, for example, a direct connection orwirelessly. Also, as indicated above, sensor 215 can be integrated withRFID tag 220.

RFID tag 220 is communicatively coupled with a transceiver 225 as shownby a line 202. RFID tag 220 can transmit data such as sensor data totransceiver 225. RFID tag 220 can transmit data to transceiver 225wirelessly. Transceiver 225 can be incorporated into a form that iscompact, such as a hand-device and can be disposed in any locationwithin the range of RFID tag 220. For example, transceiver 225 can belocated in a room where stents are stored, in a refrigerator or freezerwhere stents are stored, within a cargo container containing stents thatare being shipped, or within a truck, airplane, or train. The range candepend upon whether RFID tag 220 is an active, passive, or semi-passivetag.

Transceiver 225 can transmit data it receives to storage and processingsystem 230, as shown by a line 227. System 230 can be any type of devicecapable of storing and/or processing data received from transceiver 225.For example, system 230 can be a desktop, laptop, mainframe, PDA, etc.Transceiver 225 can be located adjacent to or integrated into system230. Alternatively, system 230 can be located in a remote location fromtransceiver 225. System 230 can be located at another location in thesame city, in another state, or another country.

Maximum desired values or tolerances for monitored environmentalparameters can be stored on, for example, RFID tag 220 or system 230. Acondition, such as temperature, monitored by sensor 215 can be comparedduring specific times or at regular intervals to the tolerance value onRFID tag 220 or system 230. If the condition exceeds a tolerance for aparameter a signal can be automatically generated by RFID tag 220 orsystem 230. If the signal is generated by RFID tag 220, the signal canbe transmitted to transceiver 225 for storage and processing on system230. The signal can be automatically transmitted to transceiver 225 toalert potential users that a stent may or is defective. A flag can bestored on RFID tag 220 indicating that a tolerance has been exceeded. Atany time subsequent to exposure to a tolerance level, a flag can be readfrom RFID 220 using transceiver 225.

Since the change in properties of a polymer are both time andtemperature dependent, a time-temperature tolerance, a maximum timeabove a selected temperature, can also be stored. For example, a flagcan be generated if a measured temperature exceeds a temperature that isa certain number of degrees below the Tg. If the time of exposure to agiven temperature exceeds a time-temperature tolerance, a separatesignal can be automatically generated by RFID tag 220 or system 230.

Sensor 215 can measure a parameter continuously over regularly spacedtime intervals, such as seconds, minutes, days, etc. Sensor 215 can alsobe programmed to measure a parameter at specific points in time, inparticular, during periods when extreme conditions can occur. Forexample, temperature measurements can be made during e-beamsterilization and shock and vibration measurements can be made duringtransport.

RFID tag 220 can be configured to store or transmit some or all of themeasurements. Measurement data from RFID tag 220 can be transmittedcontinuously as measurements are made, over regularly spaced timeintervals, or at particular points in time, such as in a manual scan ofRFID tag 220 by transceiver 225. In one embodiment, a complete historyof measurements of one or more environmental parameters can be stored ona database on RFID tag 220 or on system 230. In an embodiment, little orno measurement data is stored on RFID tag 220 or system 230. In thiscase, it may be desirable for RFID tag 220 to transmit data only when amaximum tolerance is exceeded.

In addition, numerous other types of data relating to stent 205 can bestored on RFID tag 220. Such data can include, but is not limited to,identification data, type of stent, date of manufacture, expirationdate, and place of manufacture. Such information can be obtained eithermanually or automatically from RFID tag 220 using transceiver 225. Inone useful embodiment, prior to an implant into a patient, a packagedstent can be scanned by a transceiver to determine whether the stent isdefective and whether the stent is the correct stent for the implantprocedure.

Additionally, information regarding various stents can be stored onsystem 230. This information can include expiration date and recalldata. Upon scanning by a transceiver, such information can be comparedto information relating to stent 205 stored on RFID tag 220. Forexample, identification data or date of manufacture can be used todetermine if a stent is past its expiration date and should no longer beused. Also, the identification information of a stent can be used todetermine if a stent has been recalled.

Sterilization is typically performed on medical devices, such as stents,cathethers, stent-catheter assemblies to reduce the bioburden on thedevice. Bioburden refers generally to the number of microorganisms withwhich an object is contaminated.

Radiation sterilization is well known to those of ordinary skill theart. Medical devices composed in whole or in part of polymers can besterilized by various kinds of radiation, including, but not limited to,electron beam (e-beam), gamma ray, ultraviolet, infra-red, ion beam,x-ray, and laser. A sterilization dose can be determined by selecting adose that provides a required reduction in bioburden.

However, it is known that radiation can alter the properties of thepolymers being treated by the radiation such as e-beam radiation.High-energy radiation tends to produce ionization and excitation inpolymer molecules. These energy-rich species undergo dissociation,abstraction, and addition reactions in a sequence leading to chemicalstability. The stabilization process can occur during, immediatelyafter, or even days, weeks, or months after irradiation which oftenresults in physical and chemical cross-linking or chain scission.Resultant physical changes can include embrittlement, discoloration,odor generation, stiffening, and softening, among others.

As indicated above, polymer properties are particularly susceptible tochanges in temperature. The rise in temperature is dependent on thelevel of exposure. A stent-catheter assembly can be exposed to e-beamradiation as high as 50 kGy during sterilization. As discussed above,increases in temperature of a polymeric stent can result in cracking ofstruts during deployment due to onset of brittle behavior. The increasein temperature can increase the release rate of drug resulting in adecrease of drug loading on a stent. Additionally, the deterioration ofperformance of polymeric materials and drugs due to e-beam radiationsterilization has been associated with free radical formation in adevice during radiation exposure and by reaction with other parts of thepolymer chains. The reaction is dependent on e-beam dose and level oftemperature. Therefore, it is important to have knowledge of thetemperature exposure of a stent before, during, and after a radiationsterilization process.

FIG. 3 depicts a schematic embodiment of an e-beam sterilization system300. System 300 includes an e-beam source 305 that emits an e-beam 310.A conveyer belt 315 moves as shown by an arrow 320. A stent 322 isdisposed in a container 325 which is being moved by conveyer belt 315 asshown by an arrow 330.

Container 325 is moved through e-beam source 305 to expose stent 320 toe-beam 310. Container 325 has a temperature sensor 335 and an RFID tag340 that is configured to receive temperature data from sensor 335. RFIDtag 340 can transmit temperature data to transceiver 345 located beforestent 320 passes through e-beam 310. Transceiver 345 can transmit thesensor data to a device (not shown) for storage or processing.Transceiver 345 can obtain temperature data from RFID tag 340 afterstent 320 passes through e-beam 310. Transceivers can be configured toreceive temperature data at any point along the conveyer. If themeasured temperature exceeds a tolerance allowed during or afterexposure to the e-beam, a signal can be generated by RFID tag 340, atransceiver, or a device communicating with a transceiver.

In some embodiments, container 325 with stent 322 can be cooled, e.g.,in a refrigerator or freezer, prior to e-beam exposure. A transceivercan monitor the temperature of stent 322 during the cool down processprior to e-beam exposure to ensure the temperature of stent 322 isreduced sufficiently prior to e-beam exposure. The reduced temperaturecan be less than 10° C., 0° C., −15° C., −25° C., −40° C., −70° C.,−100° C., −150° C., −200° C., −240° C., or less than −273° C. Thus,system 300 can provide documented assurance that every unit has beencooled down properly prior to sterilization.

A polymer for use in fabricating an implantable medical device, such asa stent, can be biostable, bioabsorbable, biodegradable or bioerodable.Biostable refers to polymers that are not biodegradable. The termsbiodegradable, bioabsorbable, and bioerodable are used interchangeablyand refer to polymers that are capable of being completely degradedand/or eroded when exposed to bodily fluids such as blood and can begradually resorbed, absorbed and/or eliminated by the body. Theprocesses of breaking down and absorption of the polymer can be causedby, for example, hydrolysis and metabolic processes.

It is understood that after the process of degradation, erosion,absorption, and/or resorption has been completed, no part of the stentwill remain or in the case of coating applications on a biostablescaffolding, no polymer will remain on the device. In some embodiments,very negligible traces or residue may be left behind. For stents madefrom a biodegradable polymer, the stent is intended to remain in thebody for a duration of time until its intended function of, for example,maintaining vascular patency and/or drug delivery is accomplished.

Representative examples of polymers that may be used to fabricate animplantable medical device include, but are not limited to,poly(N-acetylglucosamine) (Chitin), Chitosan, poly(hydroxyvalerate),poly(lactide-co-glycolide), poly(hydroxybutyrate),poly(hydroxybutyrate-co-valerate), polyorthoester, polyanhydride,poly(glycolic acid), poly(glycolide), poly(L-lactic acid),poly(L-lactide), poly(D,L-lactic acid), poly(L-lactide-co-glycolide);poly(D,L-lactide), poly(caprolactone), poly(trimethylene carbonate),polyethylene amide, polyethylene acrylate, poly(glycolicacid-co-trimethylene carbonate), co-poly(ether-esters) (e.g. PEO/PLA),polyphosphazenes, biomolecules (such as fibrin, fibrinogen, cellulose,starch, collagen and hyaluronic acid), polyurethanes, silicones,polyesters, polyolefins, polyisobutylene and ethylene-alphaolefincopolymers, acrylic polymers and copolymers other than polyacrylates,vinyl halide polymers and copolymers (such as polyvinyl chloride),polyvinyl ethers (such as polyvinyl methyl ether), polyvinylidenehalides (such as polyvinylidene chloride), polyacrylonitrile, polyvinylketones, polyvinyl aromatics (such as polystyrene), polyvinyl esters(such as polyvinyl acetate), acrylonitrile-styrene copolymers, ABSresins, polyamides (such as Nylon 66 and polycaprolactam),polycarbonates, polyoxymethylenes, polyimides, polyethers,polyurethanes, rayon, rayon-triacetate, cellulose, cellulose acetate,cellulose butyrate, cellulose acetate butyrate, cellophane, cellulosenitrate, cellulose propionate, cellulose ethers, and carboxymethylcellulose.

Additional representative examples of polymers that may be especiallywell suited for use in fabricating an implantable medical deviceaccording to the methods disclosed herein include ethylene vinyl alcoholcopolymer (commonly known by the generic name EVOH or by the trade nameEVAL), poly(butyl methacrylate), poly(vinylidenefluoride-co-hexafluororpropene) (e.g., SOLEF 21508, available fromSolvay Solexis PVDF, Thorofare, N.J.), polyvinylidene fluoride(otherwise known as KYNAR, available from ATOFINA Chemicals,Philadelphia, Pa.), ethylene-vinyl acetate copolymers, and polyethyleneglycol.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects.

1. A method of monitoring a stent comprising: obtaining readings of anenvironmental parameter from a sensor adjacent to a stent, the sensorpositioned within or on a container including the stent; andtransmitting the readings from an RFID tag located within or on thecontainer to a transceiver, wherein a maximum tolerance of the stent forthe environmental parameter is stored on the RFID tag; and comparing thereadings to the maximum tolerance for the environmental parameter. 2.The method of claim 1, wherein the readings are obtained at specifictimes or regular time intervals.
 3. The method of claim 1, wherein thereadings are obtained and stored on the RFID tag at specific times orregular time intervals.
 4. The method of claim 1, wherein theenvironmental parameter is selected from the group consisting oftemperature, humidity, vibration, and shock.
 5. The method of claim 1,wherein the readings are obtained during storage of the stent,transportation of the stent, or both.
 6. The method of claim 1, whereinwhen the comparison shows that a reading for the environmental parameterexceeds the maximum tolerance for the environmental parameter, a signalis generated indicating that the maximum tolerance for the environmentalparameter has been exceeded.
 7. The method of claim 6, wherein a flagindicating that the maximum tolerance for the environmental parameterhas been exceeded is stored on the RFID tag.
 8. The method of claim 6,wherein the signal is transmitted to a transceiver located within rangeof the RFID tag.
 9. The method of claim 8, wherein the signal istransmitted to a processing system for storage.
 10. A method ofmonitoring a stent comprising: obtaining readings of an environmentalparameter from a sensor adjacent to a stent, the sensor positionedwithin or on a container including the stent, wherein the readings arereceived by an RFID tag located within or on the container; andcomparing the readings received by the RFID tag to a maximum tolerancefor the environmental parameter.
 11. The method of claim 10, wherein themaximum tolerance for the environmental parameter is stored on the RFIDtag.
 12. The method of claim 10, wherein the maximum tolerance for theenvironmental parameter is stored on a processing system, wherein theRFID tag is communicatively coupled to a transceiver that transmits thereadings to the processing system.
 13. The method of claim 10, whereinthe comparing is performed at specific times or at regular intervals oftime.
 14. The method of claim 10, further comprising transmitting thereadings from the RFID tag to a transceiver located within range of theRFID tag.
 15. The method of claim 10, wherein if the comparison showsthat a reading of the parameter exceeds the maximum tolerance for theenvironmental parameter, a signal that a reading has exceeded themaximum tolerance for the environmental parameter is automaticallygenerated.
 16. The method of claim 15, wherein the signal is generatedby the RFID tag.
 17. The method of claim 16, wherein the signal istransmitted to a processing system, wherein the RFID tag iscommunicatively coupled to a transceiver that transmits the readings tothe processing system.
 18. The method of claim 15, wherein the signal isgenerated and stored on a processing system, wherein the RFID tag iscommunicatively coupled to a transceiver that transmits the readings tothe processing system.
 19. The method of claim 10, wherein the readingsare obtained during storage of the stent, transportation of the stent,or both.